1. Field of the Invention
The present invention is directed to phosphoric acid fuel cells, and more particularly, to a phosphoric acid fuel cell with an improved electrolyte,
In a fuel cell, chemical energy of gaseous fuel is converted into electrical power. A conventional fuel cell comprises an anode, a cathode, and an electrolyte impregnated matrix disposed between the electrodes.
In the operation of a phosphoric acid fuel cell, a hydrogen containing gas is fed to the anode and an oxygen containing gas to the cathode. The gases diffuse through the electrodes and react with the electrolyte in the presence of a catalyst on the electrodes to yield water, heat and electrical energy.
At the anode, hydrogen in the feed gas gives up electrons by electrochemical oxidation reaction. The electrical current so generated is conducted from the anode through an external circuit to the cathode. At the cathode, the electrons are then electrochemically combined with oxygen. A flow of ions through the electrolyte completes the circuit.
The electrochemical reactions proceeding in a phosphoric acid fuel cell are thus: EQU H.sub.2 .fwdarw.2H.sup.+ +2e.sup.-
at the anode, and EQU 1/2O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O
at the cathode.
Phosphoric acid is the conventional electrolyte for acidic hydrogen-oxygen fuel cells because of its high stability at elevated temperatures, the ease of removal of CO.sub.2 and water, and its high ionic conductivity. However, phosphoric acid has several undesirable properties. Its low oxygen solubility at high acid concentrations and relatively slow oxygen reduction rate lead to a very high polarization of the cathode.
2. Description of the Related Art
To enhance the oxygen reduction rate and thereby improve the efficiency of phosphoric acid fuel cells, it has been suggested to substitute trifluoromethanesulphonic acid for the phosphoric acid electrolyte in the cells.
The oxygen reduction rate in trifluoromethane-sulphonic acid is substantially higher than in phosphoric acid because of the higher oxygen solubility resulting in improved limiting current of the cell (R. C. Bhardway, N. G. Smart and J. O Bockris; Proc. Intersot. Energy Convers. Eng. Conf., 1991, Vol. 26, No. 3; pages 546-569; P. Zellenay, B. R. Scharifker and J. O. Bockris; J. Electrochem. Soc., 1986, Vol. 133, No. 11; pages 2262-2267).
Despite of its better performance, the use of trifluromethanesulphonic acid is limited to temperatures below the operation temperature of practical phosphoric acid fuel cells, because of its high vapour pressure. As a further disadvantage of trifluromethanesulphonic acid, it absorbs very easily in the porous electrodes resulting in low gas diffusion and thereby a higher electrode polarization.